Common and relatively inexpensive photoresistors, such as of the cadmium sulfide variety, are generally comprised of a photoresistive material sprayed over a plastic substrate. Leads are brought through the plastic material and terminate on a plated electrode layer which lies on top of the resistive material. An epoxy coating typically covers the photo-active area of the device.
In such a common design, lead to lead to resistance is a function of the sheet resistivity of the photoresistive layer, the geometry of the electrode layer, and the amount of light landing on the photoactive area of the device.
While such common photoresistors enjoy a distinct price advantage, the variance in measurements from one photoresistor to the other can be significant. Plus or minus fifty percent tolerance parts are standard. Thus, the resistance measured upon exposure to a calibrated light level and color temperature may vary dramatically from part to part. This may prove unsatisfactory for certain potential uses for the parts. For instance, a light feedback system for notebook computers comprises one possible use for a photoresistor. In this use a 50% tolerance of parts is not acceptable.
This invention discloses a technique for reducing significantly the effective parts tolerance of common, relatively inexpensive photoresistors. The invention teaches trimmed photoresistors and a trim technique. Reduction of effective parts tolerance from an untrimmed plus or minus fifty percent to a trimmed plus or minus one percent has been experienced.